There are known processes for transfer of sublimation-type dye and a process for transfer of heat-melting layer containing pigment, as a process for preparing a color image or masking film by recording digital signals by means of a thermal head.
The process for transfer of sublimation-type dye comprises a step of superposing a transfer sheet comprising a support film and an image forming layer (transfer layer) containing a sublimation-type dye and a binder thereon on an image-receiving sheet; a step of imagewise heating a surface of the support film side of the transfer sheet by the means of thermal head; and a step of subliming the dye to transfer it to the image-receiving sheet, whereby forming an image on the image-receiving sheet. Performance of the process for transfer of sublimation-type dye using transfer sheets of four colors (Y, M, C, B) enables formation of a color image and the resultant image is greatly improved in image quality.
The process for transfer of sublimation-type dye, however, has the following disadvantages:
1) the sublimation-type dye employed in the process is not suitable for recording material using thermal head showing high resolution because of its low sensitivity;
2) the dye is high expensive;
3) the process utilizes both area gradation and density gradation for tone reproduction so that it is not suitable for use in the field of printing because the tone reproduction in the field of printing (e.g., color proof) is expressed by only area gradation; and
4) edge-sharpness and durability of image formed by the process are not satisfied owing to transfer mechanism utilizing sublimation of dye for formation of image.
Particularly, unsatisfied edge-sharpness brings about serious drawback in quality of a letter image.
The process for transfer of heat-fusion layer containing pigment (thermal-transfer recording process) comprises a step of superposing a transfer sheet comprising a support in the form of a film and an image forming layer (a heat-fusion transfer layer) containing a pigment and a binder thereon on an image-receiving sheet; a step of imagewise heating a surface of the support side of the transfer sheet by the means of thermal head; and a step of fusing the image forming layer in the heated portion to bond it to the image-receiving sheet, whereby an image is formed on the image-receiving sheet. Similarly in the process for transfer of heat-fusion layer containing pigment, use of transfer sheets of four colors (Y, M, C, B) having different hues from each other enables formation of a color image.
The process for transfer of the image forming layer (heat-fusion transfer layer) containing pigment has the advantages that heat-sensitivity of the image forming layer is high, materials of the image forming layer are at a low price and the formed image has high durability as compared with the above process for transfer of sublimation-type dye. However, the image obtained by the process does not sufficiently satisfy the image quality and following-up property of the image forming layer for an uneven surface, and particularly the color image obtained by the process has unsatisfactory quality.
To solve the above problems, Japanese Patent Provisional Publication No. 4(1992)-99692 has proposed, as a masking film element, use of a thermal-transfer recording material which comprises a support and an image forming layer (a heat-sensitive transfer layer) provided thereon.
In the process, the image forming layer of the heat-sensitive transfer material is imagewise heated via a surface of a thin image-receiving film, i.e., a thermal head for heating is placed on the surface of the thin image-receiving film, and the image forming layer in the heated area is transferred to the image-receiving film. Usually, the image-receiving film to which the image forming layer is transferred is employed as a masking film, because bonding strength between the image-receiving sheet and the image forming layer is greater than that between the transparent support and the image forming layer. In the process, however, the transparent support having a thick thickness on which the image forming layer in the unheated area is left is employed as a masking film in order to render handling of the masking film easy. Hence, the masking film has drawbacks that the masking portion (image portion) is apt to peel of in using or handling the film.
Further, to solve the above problems, use of a sheet which comprises a support and an image-receiving layer comprising resin or wax provided thereon as an image-receiving sheet has been proposed (Japanese Patent Provisional Publications No. 60(1985)-225793, No. 63(1988)-82786 and No. 4(1992)-131285).
Japanese Patent Provisional Publications No. 60(1985)-225793 discloses use of various waxes (e.g., wax, paraffin wax, higher fatty acid, ester thereof and ester wax) as materials of the image-receiving layer. Japanese Patent Provisional Publications No. 63(1988)-82786 discloses use of butyral resin as a material of both the image-receiving layer and the image forming layer (thermal-transfer layer). Japanese Patent Provisional Publications No. 4(1992)-131285 discloses use of a combination of a wax such as chlorinated polyolefin, paraffin wax or chlorinated products thereof and a resin such as polyvinyl chloride, polyester or polycarbonate as materials of the image-receiving layer.
In the thermal-transfer recording process using the image-receiving sheet having the image-receiving layer of resin or wax as mentioned above, the image forming layer of the transfer material is easily and accurately transferred onto the image-receiving layer (image-receiving sheet) under heating as compared with the process using the image-receiving sheet with no image-receiving layer. However, the obtained image is not sufficiently satisfied in quality due to unsatisfactory transferring property (heat-sensitivity), and otherwise the image is apt to be scratched because of low hardness of the image. In more detail, if the image-receiving layer is designed to enhance transferring property, the resultant image is apt to be scratched. In contrast, if the image-receiving layer is designed to prevent the resultant image from being scratched, the transferring property is reduced.